Harbor porpoise mothers are exposing their calves to high levels of harmful chemicals known collectively as polychlorinated biphenyls (PCBs), unwittingly detoxifying themselves when nursing their young.

Though PCBs were banned across much of the world in the 1970s and '80s, the artificial chemicals still trickle into the marine ecosystem through terrestrial runoff and atmospheric transfer. The chemicals were once used in surface coatings and paints and are now considered among the “most ubiquitous and resistant pollutants in the global ecosystem,” according to the UK Marine Special Areas of Conservation. Effects are varied depending on species and exposure levels but are largely shown to reduce reproductive capacity and impact brain development.

Researchers at Cetacean Strandings Investigation Programme (CSIP), the Zoological Society of London (ZSL), and Brunel University London now write in the journal Science of the Total Environment that at least 209 types of the PCBs are affecting marine mammals, with some types being less toxic or more efficiently metabolized than others. Harbor porpoise calves were also shown to have higher levels of neurotoxic PCBs than their mothers.

"The concentrations we observed in several of the juveniles were above proposed thresholds for toxic effects in marine mammals. Therefore, they are at levels that are associated with reduced calf survival, immunosuppression, reduced fertility, neurotoxicity as well as several other toxic endpoints," study author Rosie Williams of ZSL and Brunel University London told IFLScience. 

Scientists often monitor PCB concentrations by grouping them together and treating them as one chemical rather than as a group of chemicals with different toxicity levels, added Williams in a statement. To highlight the need for changing this approach, researchers measured individual PCB profiles in adult females and males, as well as juveniles and neonates, allowing them to identify patterns of transference. 

"For example, some PCBs had very high ratios in juveniles and very low ratios in adult females, therefore we could conclude that those PCBs had a higher affinity to being transferred from mothers to calves via their milk," she said, calling the findings a “tragic irony”. Juvenile porpoises should be receiving vital nutrients for development, but instead are being exposed to potentially dangerous chemicals.

Williams notes that studying PCB exposure in abundant species like porpoises can help to predict the effects in more vulnerable species already seeing population declines, such as a UK-based orca pod that was left with just eight individuals after a resident was killed in 2017 in Scotland. A necropsy later revealed that the individual had the highest level of toxic pollutants ever recorded in the region, with PCB levels thought to be 100 times greater than what is considered the minimum toxic level for marine mammals.  

"It's obvious that marine mammals are still experiencing the lingering impacts of PCBs, so identifying the sources and pathways they're entering our oceans is a vital next step to preventing further pollution,” Williams said.

It is important to note that the study's conclusions are limited in that only a restricted number of PCB profiles were considered. 

"Furthermore, levels have not declined at the same rate as other pollutants, and analyses have shown that levels may have stabilized," added Williams. "Despite this, it is still important to consider that harbor porpoises are exposed to a wide range of other legacy and emerging contaminants that will also affect the toxicity of pollutant burdens."

PCBs have been found in some of the deepest trenches and abyssal-living creatures of the world’s oceans, with the highest amounts shown in toothed whales, or odontocetes. A study published last fall found that the number of killer whales left in the wild could be halved within the next 30 to 50 years due to exposure to the potential harmful range of chemicals.